Gun barrels, turbine components, internal combustion engine components, aerospace components, chemical reactors, machine tools, drilling equipment, bearings and the like are often comprised of iron, steel or other ferrous alloys. In use, such articles are frequently exposed to various combinations of high temperatures, high pressures and corrosive ambient environments. These conditions can cause thermochemical erosion of the substrate materials leading to pitting, cratering, cracking and failure.
The prior art has recognized such problems and has attempted to prevent or minimize the erosion of ferrous materials by the use of various coatings comprised of high hardness materials. For example, U.S. Patent Application 2002/0104588 discloses a process for extending the life of mechanical centrifuge screens by forming a layer of high hardness iron nitride on the screen and subsequently electroplating a layer of chromium onto the nitride layer. The nitride layers of the '588 application are high hardness layers including at least 33 atomic percent nitrogen. Likewise, U.S. Pat. Nos. 5,887,558 and 5,810,947 show coatings of high hardness iron nitride used in connection with internal combustion engines and machine tools respectively. As will be explained in detail hereinbelow, such prior art methods have been found to be unsuitable for, and in some instances actually derogatory to, enhancing the thermochemical stability of steel and the like under high temperature, high pressure reactive conditions.
The present invention may be utilized to enhance the thermochemical stability of a variety of articles. For the purposes of this present discussion, the invention will be described primarily with regard to gun barrels; however, it is to be understood that the invention may be used with equal advantage in connection with any other articles which are exposed to conditions which include one or more of high temperatures, high pressures and corrosive environments. These articles include, by way of illustration and not limitation, internal combustion engine components, turbine components, aerospace assemblies, chemical reactors, machine tools, drilling equipment, bearings and the like.
Referring now to FIG. 1, there is shown a cross-sectional view of a portion of the bore of a gun barrel 10 of the prior art. The gun barrel 10 is comprised of a body of a steel alloy, and a portion of this body of steel alloy is shown at reference numeral 12. It is to be understood that in some instances gun barrels are fabricated as composite members having a steel liner which defines the gun bore, and this liner is encased in a body of another material such as a body of metal or a body of a reinforced polymer.
The gun barrel 10 shown in FIG. 1 is typical of, and representative of, barrels associated with relatively large artillery pieces as well as small arms. The barrel 10 of FIG. 1 includes a coating of chromium 14 deposited on the surface of the bore thereof. In some instances a gun barrel will have a layer of another refractory material thereatop, or it may not have any refractory material at all. The present invention may be used in any of these types of gun barrels. The chromium 14 provides a smooth, high hardness surface which minimizes wear of the barrel. As is shown in FIG. 1, the layer of chromium 14 includes a number of cracks 16a-16d defined therein. These cracks 16 pass through the layer of chromium 14 and expose portions of the surface of the underlying steel alloy 12. These cracks 16 can occur as a result of stresses which arise when the chromium is deposited, and further cracking can occur during use of the gun.
Ignition of a propellant charge creates a volume of high temperature, high pressure, combustion products which propel a projectile through the barrel. These combustion products can be in the form of ions, radicals or neutral species. The cracks 16a-16d in the chromium layer 14 will permit these combustion products to contact the underlying body of steel 12 so as to cause a chemical reaction to occur between components of the combustion products and the steel. For example, it has been demonstrated that CO, one of a number of reactive combustion products, can react with the steel of gun barrels, under firing conditions, to cause carburization of the steel.
As shown in FIG. 1, portions 18a-18c of the body of steel 12 have been carburized in the regions of cracks 16a-16d. Such reactions can adversely change the properties of the steel. For example, typical gun steel has a melting point of approximately 1723° K; however, if the steel is carburized its melting point drops to 1423° K. The lowering of the melting point makes carburized portions of the barrel prone to pitting and other erosion as a result of use of the barrel. Such erosion can spread beneath the chromium layer, as is specifically shown for region 18b, thereby causing portions of the chromium layer to form new cracks and/or flake away from the surface of the barrel. This will expose further portions of the steel to the propellant gas leading to further carburization and erosion. Similar reactions can also occur in engines, turbines and the like under high temperature and/or high pressure conditions.
Clearly, there is a need for structures and methods for stabilizing steel alloys against thermochemical corrosion which can occur under severe use conditions. Any such structure and method should be simple to implement and should not interfere with the function of the item. As will be explained in greater detail hereinbelow, the present invention provides such structures and methods.